Interferometry (e.g., low coherence interferometry) may be used to determine a spatial property of an object. Typical spatial properties include a surface topography or location of the object with respect to some reference. For objects including a thick film overlying an opaque substrate, the low coherence interferometry data may include two spaced part interference patterns resulting, respectively, from the substrate-film interface and film-air interface. The accuracy of interferometry depends on the quality of the interference signal, which can be degraded by several sources of error, many of these systematic and related to the limitations of the instrument.